UPON INSTRUMENTS CALLED PERISCOFIC. 103 



the focal distance of oblique pencils is not merely as great, but 

 is greater than that of a direct pencil. For, since the effect of 

 tbfl first surface is to occasion divergence of parallel rays, and 

 thereby to elongate the focus ultimately produced by the second 

 gurface, and since the degree of that divergence is increased by 

 obliquity of incidence, the focal length resulting from the com* 

 bined action of both surfaces will be greater than in the centre, 

 if the incidence on the second surface be not so oblique as to 

 increase the convergence. On this account, the opening E is 

 placed so much nearer to the lens than the centre of its second 

 surface, that oblique rays Ef, alter being refracted at the first 

 surface, are transmitted 'hrough the lens neatly in the direction 

 of its shorter radius \ .and hence are made to converge to a 

 point so distant, that the image (at/) falls very nearly in the 

 same plane with that of an object centrally placed." 



The radii of curvatures for a meniscus of 22 inches focus , Observations 

 being as two to one, is not essential. The theory of dioptrics ££ mtDi%ctu iM 

 shews, the greater the pioportion, or the nearer that the radius inferior in its' 

 of one side approaches to infinity, or a piano, the more perfect * ^Mgconvex 

 the lens will be. Dr. W. has not stated the diameter of the lens, 

 convex lens, but the reader must suppose it to be four inches, 

 like that of the meniscus ; nor has he told the reader what im- 

 provement would be produced, if he placed a similar circular 

 Opening, or limited aperture, also over the convex lens. I 

 must, therefore, inform the reader, and he may himself prove 

 it to be correct. The diameter or aperture of four inches is 

 too great for a lens of 22 inches focus, either double convex, 

 or meniscus lens, placed in a Camera Obscura, as it transmits 

 too much light, and produces too much aberration for the most 

 distinct representation of the images within the Camera. Dr, 

 W., therefore, no doubt, was obliged to correct this palpable 

 defect, by a curtailment of the area of his lens no less than 

 three-fourths of the whole, and the 1 ns would have been more 

 like one applied by a skilful optician, if he had at first inserted 

 a lens of about two inches diameter. The limited aperture, 

 therefore, it is evident, advantageously excludes superfluous 

 rays, but has nothing to do with the determination of their 

 direction. Upon a fair comparison, the reader will not only 

 doubt of the superiority of Dr. W.'s Camera, but be convinced 

 of its absolute inferiority 5 for the double convex lens, under the 



same 



